Coseismic Dehydration and Amorphisation of Serpentinite in a Creeping Shear Zone

Recent experiments show that coseismic heating in serpentinite-bearing faults can produce dehydration assemblages consisting of olivine + enstatite or talc, as well as amorphous material and melt. Identification of these coseismic products has not been made in natural serpentinite shear zones, possibly because the reaction products would be quickly altered or rehydrated to form serpentine. Magnetite-coated slip surfaces within the serpentinite mélange of the Livingstone Fault, New Zealand, contain microstructural and mineralogical evidence for coseismic dehydration and amorphization of serpentine due to frictional heating. The bulk serpentinite mélange has a scaly fabric that contains abundant evidence for pressure-solution creep. These fabrics are crosscut by multi-layered, magnetite-coated slip surfaces up to 1.5 mm thick that enclose numerous thin (<100 µm) selvages of serpentinite. Newly-developed techniques of sub-micron confocal Raman Spectroscopy combined with TEM examination show that the selvages of serpentinite are loaded with aggregates of nanocrystalline olivine, enstatite and talc, as well as amorphous pure silica and poorly-crystalline serpentine. The highest temperature products (nanocrystalline olivine, enstatite, amorphous silica) are restricted to a zone <100 µm wide adjacent to the main slip surface. Further from the slip surface, nanocrystalline olivine co-exists with talc, grading into amorphous and poorly-crystalline material. At distances > 500 um from the main slip surface, no products of dehydration or amorphisation are identifiable. Encapsulated within the magnetite layers and protected from subsequent hydration, the thin selvages of serpentinite provide the first evidence of highly localised dehydration and amorphisation of serpentinite in a natural shear zone. Interpreted in the context of recent experiments, our results suggest that 1) the slip surfaces experienced extreme dynamic weakening associated with coseismic dehydration and amorphisation and, 2) localized rupture and coseismic slip can propagate through serpentinite mélange deforming by bulk pressure-solution creep.